Identifying Ions

Some solid metal ions produce coloured flames when you heat them in a Bunsen burner. Many solutions containing metal ions undergo reactions where a precipitate is produced. We can use this knowledge to identify unknown metal ions in the lab.

 
 

Flame tests

A flame test is carried out by cleaning a platinum or nichrome wire by placing it in concentrated hydrochloric acid. We use platinum or nichrome because they have a high melting point (so it won’t melt in the flame) and are unreactive. We place the acid-soaked wire into the centre of a hot Bunsen flame then place it into a sample of the metal we want to test. We then place it back into the non-luminous flame - it needs to be non-luminous (not very orange) so we can clearly see the colour.

  • Lithium, Li+ burns with a red/crimson flame
  • Sodium, Na+ burns with a yellow-orange flame
  • Potassium, K+ burns with a lilac flame
  • Calcium, Ca2+ burns with an orange-red flame
  • Copper, Cu2+ burns with a green flame
 
 

Metal hydroxides

We can also use sodium hydroxide solution to identify metal ions (cations). When you add sodium hydroxide to solutions containing aluminium ions (Al3+), calcium ions (Ca>sup>2+) or magnesium ions (Mg2+), white precipitates are formed. This is because aluminium hydroxide, calcium hydroxide and magnesium are all insoluble white solids. These reactions are examples of displacement reactions.

Of the three hydroxides, only aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution. This means that if you have a solution containing an unknown cation, you could test it by adding a small amount of sodium hydroxide. If a white precipitate is formed which is able to dissolve in excess sodium hydroxide solution, then you know that the solution contained aluminium ions.

Solutions containing copper (II) ions (Cu2+), iron (II) ions (Fe2+) and iron (III) ions (Fe3+) form coloured precipitates when sodium hydroxide solution is added. Copper (II) forms a blue precipitate of copper (II) hydroxide, iron (II) forms a green precipitate of iron (II) hydroxide and iron (III) forms a brown precipitate of iron (III) hydroxide.

From left to right: iron (II) hydroxide, iron (III) hydroxide and copper hydroxide. Image: Kingston Grammar School

Carbonates

You can test to see if a compound is a carbonate by reacting it with a dilute acid.

If the substance is a carbonate, carbon dioxide gas will be formed which can be identified by bubbling through limewater. If the limewater turns milky then the gas is carbon dioxide.

 
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Halides

We can test for the halide ions (the ions formed from the group 7 elements) using dilute nitric acid followed by silver nitrate. We add dilute nitric acid to remove any carbonate or sulfate ions which would give a false positive result. Silver ions react with the halide ions to form precipitates of silver chloride, silver bromide or silver iodide which deepen in colour from white, cream then yellow. The equations for these reactions are:

Sulfates

Sulfate ions are tested by adding dilute hydrochloric acid followed by barium chloride. The purpose of the acid is to remove any carbonate ions that may give a false positive result. The reaction between sulfate ions and barium chloride forms barium sulfate which is an insoluble white precipitate.

Instrumental methods

Besides carrying out a chemical test to identify compounds, instrumental methods can be used to identify substances. Instrumental methods use machines for identification, and include technology such as flame emission spectroscopy. Compared to chemical tests, instrumental methods are:

  • More accurate

  • More sensitive

  • Rapid

Flame emission spectroscopy

Flame emission spectroscopy is an example of an instrumental method used to analyse metal ions in solutions. It involves the following steps:

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  1. The sample is put into a flame

  2. The light given out is passed through a spectroscope

  3. The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.

Because different types of metal ion will produce different spectra, you can just compare the line spectrum with those that have been published in a data book to identify the metal ion. In order to measure the concentration, you would first make a calibration curve by taking flame photometer readings of different metal ion concentrations. You can then plot a graph of photometer readings on the y-axis and metal ion concentration on the x-axis (this is the calibration curve). You then read off your calibration curve at the photometer reading of your sample to determine its concentration.

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Did you know…

The Northern lights (aurora borealis) are caused by photons released by oxygen and nitrogen in the air. Wind particles hit atoms of oxygen and nitrogen, causing them to gain energy and become excited. When the molecules return to their normal (ground) state, they release light energy of different colours.